This invention relates to methods for determining the thickness of both opaque or transparent samples (e.g., thin films).
Thin films of dielectric (e.g., polymer, oxide) and conducting (e.g., metal) materials are used in a range of microelectronic, optical, and biomedical devices. A microprocessor, for example, contains multiple layers of metal, semiconducting, and oxide thin films. Thickness variations in these films can modify the films"" electrical and mechanical properties, thereby affecting the performance of the microprocessor. Accordingly, film thickness is often monitored as a quality-control parameter during and/or after the microprocessor""s fabrication.
Well-known techniques, such as optical ellipsometry and interferometry, measure the thicknesses of transparent films. Measuring the thicknesses of opaque films (e.g., metal films) is a more difficult problem. Current methods for measuring metal film thickness include electrical tests and x-ray spectroscopy. In the electrical test (commonly called a sheet-resistance test), a pair of conducting probes contact the film; electrical resistance, as measured by the probes, is proportional to the film""s thickness. Sheet-resistance tests require contacting the film, making such tests undesirable for measuring actual devices during the fabrication process. X-ray-based techniques measure the thickness of metal films by inducing, measuring, and analyzing x-ray fluorescence. This method is both non-contact and non-destructive, but requires large, unwieldy instruments that generate hazardous x-ray radiation.
In general, in one aspect, the invention provides a method for determining the thickness of both transparent or opaque samples (e.g., thin films within a microelectronic circuit). The method includes the step of exciting time-dependent xe2x80x9cacoustic waveguide modesxe2x80x9d in the sample using an excitation radiation field. The radiation field includes a spatially and temporarily varying optical interference pattern containing alternating xe2x80x9cbrightxe2x80x9d and xe2x80x9cdarkxe2x80x9d regions defined by a wavevector (described in detail below). Absorption of light in the bright regions launches acoustic waveguide modes that induce a ripple morphology an the sample""s surface. The waveguide modes are measured by diffracting a portion of the probe radiation off the ripple morphology. The diffracted probe radiation is then detected and analyzed to yield the frequency of the acoustic waveguide modes. This property, in turn, is related to the sample""s thickness.
In another aspect, the invention provides a method for determining the thickness of a thin sample by exciting and detecting time-dependent acoustic waveguide modes in the sample. Here, to increase the accuracy of the measurement, the wavevector of the acoustic waveguides modes is chosen to optimize (e.g., maximize or nearly maximize) the dependence of the frequency on the thickness of the sample. The thickness of the sample is then determined from the frequency.
Several techniques are used to analyze the acoustic waveguide modes to determine the sample""s thickness. For example, the acoustic frequencies can be compared to a database that correlates frequency to sample thickness. This type of database is generated by measuring and recording the acoustic frequencies from standards that include films having known thicknesses.
Alternatively, the sample""s thickness is determined by comparing the acoustic properties to a mathematical model. The model describes the stiffness (i.e., elastic moduli) and thickness of each layer in a multi-layer film/substrate structure, and can be used to analyze single or multiple-frequency measurements. For the multiple-frequency measurements, the exciting, detecting, and analyzing steps are repeated to generate a xe2x80x9cdispersionxe2x80x9d containing multiple frequencies, each corresponding to a different wavevector. The determining step of the method is an iterative fitting procedure that includes: i) proposing initial components of a stiffness tensor and an initial thickness of the sample; ii) calculating an initial dispersion based an the proposed initial components of the stiffness tensor and the initial thickness; iii) comparing the calculated dispersion to the measured dispersion; iv) repeating the proposing, calculating, and comparing steps until the calculated dispersion matches the measured dispersion to a desired degree; and, v) determining the thickness of the sample from the calculated dispersion. If the components of the stiffness tensor are known, only the thickness-needs to be varied during the fitting procedure. Using the above-mentioned determining step, a single frequency can be compared to a model of a single or multi-layer film/substrate structure to calculate film thickness.
Any property that can be calculated from the frequency can be used in place of frequency during the steps described above. For example, a phase velocity (i.e., the frequency divided by the excitation wavevector) can be used in the calculations.
Samples having a range of thicknesses and compositions can be measured with the method of the invention. The sample can be, for example, thin films of metals, polymers, semiconductors, or oxides having thicknesses ranging from 100 angstroms to 20 microns. The product of the wavevector of the excitation field and the thickness of the sample is typically between about 0.1-10. The sample can be included in a device as an xe2x80x9couterxe2x80x9d layer (i.e., having an exposed planar surface) or xe2x80x9cunderlyingxe2x80x9d layer (i.e., having no exposed planar surfaces) in a film/substrate structure.
The method of the invention is carried out using a film-measuring instrument that includes excitation and probe lasers for generating, respectively, a pulse of excitation radiation (at a wavelength that is strongly absorbed by the sample) and probe radiation (at a wavelength that is not substantially absorbed by the sample). Each pulse of the excitation radiation is typically focused as an elliptical spot having a major axis along the direction of the wavevector; optical interference between these spots generates the spatially and temporally varying excitation field. The probe radiation spot is typically round and covers an area which is about an order of magnitude smaller than the excitation field spot size. Each excitation pulse typically has a duration less than 1 nanosecond, while the probe radiation is typically cw radiation or a pulse having a duration longer than a lifetime of the acoustic waveguide mode (e.g., greater than about 500 nanoseconds).
The film-measuring instrument includes an optical system oriented to receive the pulse of radiation from the excitation laser, and then separate the pulse into at least two optical pulses. The system delivers the pulses to the sample so that they interfere and form the optical interference pattern within or on top of the sample. This launches the acoustic waveguide modes which, in turn, generate the ripple morphology an the sample""s surface. The probe radiation is oriented to diffract off of the ripple morphology to form a diffraction signal which is then monitored with an optical detector (e.g., a fast photodiode having a response time faster than 1 nsec). In this way, a light-induced signal indicating the frequency or phase velocity of the acoustic waveguide mode is generated and analyzed to determine a thickness of the thin sample.
The excitation laser is a typically a Nd:YAG, Nd:YLF, or nitrogen laser. The probe laser is typically a cw laser, such as a gated diode laser.
The invention has many advantages. For example, the film-measuring instrument can be a stand-alone unit, or can be attached directly to a film-formation tool (e.g., a chemical-vapor deposition tool, a plasma-vapor deposition tool, a cluster tool, or a vacuum chamber) or a film-processing tool (e.g., a chemical-mechanical polisher). In this embodiment, the film-formation tool includes an optical port (e.g., a glass window) that is transparent to the excitation and probe radiation. Thus, during operation, the film-measuring instrument is oriented so that the excitation and probe radiation, and the diffraction signal, pass through the optical port. In this mode of operation, the instrument remotely measures films undergoing processing at both high and low temperatures. The instrument also functions in-line during semiconductor wafer processing to determine the exact thicknesses-of metal, oxide, polymer, and semiconductor films, or to determine if thicknesses of these films are out of specification. In one embodiment, for example, the film-measuring instrument alerts a process controller when unacceptable films are being fabricated, and sends signals that correct fabrication parameters (e.g., temperature). This process can be repeated iteratively until films having the desired thickness are formed. Because film-formation processes are directly and non-invasively monitored, the instrument increases device yields, removes defective product from the fabrication process, and, in general, facilitates production of high-quality films.
The film thickness-measuring instrument collects practically noise-free data in a real-time, non-destructive and non-contact manner. Data from a single measurement point (having an area of between 10 and 100 microns) are typically collected in less than one second and used to determine a film""s thickness to within tens of angstroms. For typical film thicknesses (e.g., 1 micron) this corresponds to a fraction of a percent of the film""s true value. The instrument is also compact, occupying a footprint of about 2 square feet.
Other, features, aspects, and advantages of the invention follow from the following detailed description, and from the claims.